The present relates to a method and circuit for electronically filtering a signal. The invention is particularly useful in Doppler motion sensor devices, such as used in intrusion detector systems, and is therefore described below with respect to such an application, but it will be appreciated that the invention, or various aspects thereof, can also be used in many other applications.
Intruder detector systems are generally based on infrared radiation devices and/or microwave or ultrasonic Doppler devices, for sensing the motion of an object within the protected space. Infrared radiation devices sense the motion of a heat source, whereas Doppler devices sense the motion of physical masses which reflect the microwave or ultrasonic waves. Both types of devices tend to produce false alarms which, if occurring too frequently, can affect the integrity of the intrusion detector system. Accordingly, in some applications, the intrusion detector system may include both types of devices both of which must be actuated to actuate the alarm in order to minimize false alarms.
Since the Doppler device is based on reflecting microwaves or ultrasonic waves from an object within the monitored space, such devices are particularly prone to the production of false alarms by various types of moving objects within the monitored space. Particularly troublesome are electrical devices, such as a fluorescent lights, energized by the electrical supply mains. Thus, in a fluorescent light, the gas within the tube produces a moving xe2x80x9cheat frontxe2x80x9d from one end of the tube towards the opposite end with each ignition of the tube, such as to simulate a moving heat source which could be incorrectly interpreted by the intrusion detector system as a moving intruder.
As the frequency of the supply mains is known, one way of avoiding this source of false alarms is by subtracting the line frequency from the signal outputted by the Doppler detector device. However, harmonics of the line frequency are also generated by the fluorescent light, and harmonics can also produce a false alarm. Therefore, it would be necessary to make a computation and subtraction for each such harmonic, which is a costly and time consuming process. Moreover, the interference signal may be so strong as to mask the true signal.
Another source of false alarms particularly in Doppler intrusion detector systems are objects, such as fans, moving at a relatively constant velocity within the monitored space. The motion of such devices can also be incorrectly interpreted as an intruder to produce a false alarm.
An object of the present invention is to provide a filtering method and circuit particularly useful in microwave or ultrasonic Doppler detector systems for reducing one or both of the above sources of false alarms in such systems. Another object of the present invention is to provide a Doppler motion detector system, and also an intrusion detector system, having advantages in the above-described respects.
According to one aspect of the present invention, there is provided a method of filtering an input signal which includes noise cyclically repeating at a known noise frequency, to substantially remove said noise from the input signal, comprising: sampling the input signal at a frequency corresponding to a whole multiple xe2x80x9cNxe2x80x9d of the noise frequency; sequentially storing the samples in 0-N storage devices; sequentially subtracting the sample in each storage device from the sample previously stored in the Nth storage device preceding the respective storage device, to thereby produce for each sample, a difference sample in which the cyclically repeating noise is effectively cancelled from the respective sample; and sequentially outputting the difference samples to produce an output signal from which the cyclically repeating noise has been substantially removed.
According to further features in the described preferred embodiment, xe2x80x9cNxe2x80x9d is preferably a whole number greater than xe2x80x9c1xe2x80x9d such that the harmonics of the cyclically-repeating noise are also substantially removed. For example, where the supply line is at a frequency of 50 Hz, xe2x80x9cNxe2x80x9d is preferably xe2x80x9c12xe2x80x9d, whereupon the sampling frequency would be 600 Hz; and as will be shown below, such a sampling frequency will be effective with respect to the fundamental (the line) frequency of 50 Hz, and also with respect to its harmonics 100 Hz, 150 Hz and 200 Hz. Similarly, if the line frequency is 60 Hz and xe2x80x9cNxe2x80x9d is 12, the sampling frequency would be 720 Hz, whereupon the filtering circuit would be effective with respect to the corresponding line frequency and its harmonics.
The novel method, therefore, does not require computing and subtracting each harmonic, nor the costly and time-consuming procedure that would be involved. Moreover, the novel method removes the cyclically repeating noise even where that noise is so strong that it might otherwise tend to mask the true signals.
According to further features in the preferred embodiment described below, the input signal being filtered is the output of a microwave or ultrasonic Doppler motion sensor device in an intrusion detector system. When used in such a system, other operations may be performed, in addition to or in lieu of the foregoing operations. In order to reduce the possibility of constantly-moving objects, such as fans, within the monitored space producing a false alarm. These other operations could include the following: examining each sample of the input signal for a change in amplitude over the previous sample; for each such changes in amplitude over a threshold, measuring the duration of the change in amplitude from the time of the amplitude change until the time the amplitude of a subsequent sample input signal drops below the threshold; and maintaining an alarm signal only for those changes in amplitude having a duration shorter than a predetermined time period. In the described preferred embodiment, an alarm signal is produced whenever a change in amplitude is detected over a threshold with respect to the previous signal, and the alarm signal is terminated whenever the duration exceeds a predetermined time period.
According to another aspect of the present invention, there is provided an electronic filter for filtering an input signal which includes noise cyclically repeating at a known noise frequency, to substantially remove the noise from the input signal, comprising: means for sampling the input signal at a frequency corresponding to a whole multiple xe2x80x9cNxe2x80x9d of the noise frequency; a shift register including 0-N registers for sequentially storing the samples in the registers, and for reading them out from the registers in a FIFO manner; and a subtractor for sequentially subtracting the sample in each register from the sample previously stored in the Nth register to thereby sequentially produce, for each sample, a difference sample in which the cyclically-repeating noise is effectively cancelled from the respective sample. As will be described more particularly below, such a filter acts as a dynamic electronic filter to filter out the cyclically repeating noise from the input signal. In the described preferred embodiment, this dynamic filter is implemented by software in a processor.
According to a still further aspect of the present invention, there is provided a Doppler motion sensor device for sensing motion of an object, comprising: a transmitter for transmitting energy (microwaves or ultrasonic waves); a receiver for receiving the energy after reflection from an object; a mixer for mixing the transmitted energy and the received energy, and for outputting a signal representing the velocity of motion of an object reflecting the energy; and an electronic filter as described above for substantially removing cyclically-repeating noise from the output signal from the mixer.
The Doppler motion sensor, theoretically, could be used as a stand-alone system for detecting intrusions. However, in order to minimize the possibility of false alarms, it is preferable that the intrusion detector system includes both the above-described Doppler motion sensor device, and also an infra-red radiation motion sensor device, such that the alarm would be actuated only when both devices output an alarm signal within a predetermined time window.
Further features and advantages of the invention will be apparent from the description below.